Method and installation for the treatment of a radioactive wastes

ABSTRACT

This invention concerns a method and an installation for the treatment of radioactive wastes produced as a consequence of the operation of nuclear power plants with pressurized water reactors and boron reactivity regulation and will take place for the production of substances like borax with environmentally allowed content of radioactive isotopes that contains only the cesium isotopes with maximum total concentration of 800 Bq in one kilogram of borax; calcium, magnesium or calcium-magnesium borates with environmentally allowed content of radioactive isotopes; boron acid solution with environmentally allowed content of radioactive isotopes; sodium hydroxide solution containing only the cesium isotopes with maximum total concentration of 800 Bq in one kilogram sodium hydroxide; radioactive waste containing under 5 g/l of boron acid.

TECHNICAL AREA

[0001] The present invention relates to a method and an installation forthe treatment of radioactive wastes resulting from the operation ofnuclear power plants with pressurized water reactors using boronreactivity regulation and is destined for the yielding of products likeborax, calcium-magnesium borates, boron acid and sodium hydroxidesolution with allowed content of radioactive isotopes suitable formultiple use and for comprehensive environmental protection.

BACKGROUND OF THE INVENTION

[0002] It is known a method for the treatment of radioactive waste water(BG 32683) that is first converted to an almost dry salt and thereaftertreated with alcohols with longer carbon chain. At the treatment ofboron acid-containing salts with alcohols begin an esterification of theboron acid and alcohol. The ester so obtained is treated by hightemperature (above 100° C.) distillation until boron acid is produced.

[0003] The main shortcoming of this method is that all the processes arerunning at high temperatures, using longer carbon chain alcohol andapplying ester distillation of boron acid with large heat energyconsumption. Basically this process is very complicated and laborconsuming.

[0004] It is known also a method is known (BG 51265A) for the treatmentof radioactive wastes until the obtaining boron acid containingradionuclides with environmentally friendly concentration. The methodconsists of several stages—concentration of radioactive wastes withdifferent indications of pH—lower than 4.5 and higher than 8.5, andthereafter mixing the concentrates. Under appropriate temperature regimethe radioactive waste containing boron acid in concentration of 15-20g/l and borax in environmentally admissible radioactive isotopesconcentration is produced, which is no more a radioactive waste. Boronacid solution is obtained from the borax by means of a suitabletechnology-ion-exchanging or electrodialysis.

[0005] A shortcoming of this method is the obtaining of the radioactivewaste contains 15-20 g/l non-radioactive boron acid, subject to along-term storage. The final product contains non-radioactive acid,which fills the large capacities in the radioactive waste storagefacilities, reduces the concrete matrix strength and allows radioactiveisotopes, washing away from the matrix in case of radioactive wastecementation method application.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to establish a method andan installation for the treatment of liquid radioactive wastes, enablingthe preparation of the radioactive wastes for a long-term storage with aminimum non-radioactive boron acid content and the production ofenvironmentally harmless materials appropriate for further use.

[0007] The solution of this problem can be found through a method, bywhich after the separated collection of acid and alkali radioactivewastes, the separated concentration of radioactive wastes is broughtwith pH above 8.5 until reaching a concentration of boron acid from 35to 200 g/l —on the one hand, and on the other hand—of radioactive wasteswith pH under 6.0 not free of boron acid, until the total salt contentreaches levels between 400 and 500 g/l. Then the concentrates are mixedin ratio from 0.5:1 to 1:2 based on recalculated values of contained inthe wastes gram-equivalent ratio of sodium hydroxide to boron acid.These ratios lead to the achieving of mixture pH from 8.0 to 10.1. Ifnecessary additional pH corrective additives are being used—for examplenitrogen acid or other acid, sodium carbonate or sodium hydroxide. Theborax separation process runs under these conditions and the boron acidconcentration in the mother solution reaches 20-25 g/l. The separateborax crystals are dissolved and filtered so that the join saltdepositions be separated. From refined solution the borax recrystalliseswith content of radioactive isotopes that allow its storage in standardchemical storage conditions, e.g. radiologically safe for theenvironment.

[0008] Subsequently the borax crystals filtration process part of theobtained substance at a concentration of 20-25 g/l may be used, or thesolution with the same concentration may be prepared from separate boraxcrystals for treatment by electrodialysis until boron acid solutions inconcentration from 0.1 to 60 g/l and sodium hydroxide in concentrationup to 150 g/l are obtained. The assembly for electrodialysis operates bymeans of heat-resistant membranes and electrical current 0.2 to 45 Awith voltage from 5.0 to 55 V.

[0009] The base part of the produced radioactive waste (filtrate)containing boron acid in concentration of 20-25 g/l is treated by saltsof alkaline-earth metals in which process a non dissolvable borates areobtained. This reaction produces radioactive waste—filtrate containing2-4 g/l of boron acid.

[0010] As an alkaline-earth metals calcium and magnesium salts or theirmixtures are used.

[0011] The substances obtained through this radioactive waste refiningmethod according to this invention are as follows:

[0012] Borax with environmentally allowed content of radioactiveisotopes including only cesium isotopes with maximum total concentrationof 800 Bq per kilogram borax;

[0013] Calcium, magnesium or calcium-magnesium borates with allowed forthe environment content of radioactive isotopes;

[0014] Boron acid solution with environmentally allowed content ofradioactive isotopes;

[0015] Sodium hydroxide solution containing only the isotopes of cesiumin maximum total concentration of 800 Bq per kilogram sodium hydroxide.

[0016] Radioactive waste containing under 5 g/l boron acid;

[0017] The method according to the present invention was realized withan installation for the treatment of a radioactive waste.

[0018] This installation includes reactor-homogenizer, fed by reservoirsfor radioactive wastes connected to it, with pH under 5.5 and pH above5.5, as well as reservoir for pH correction additives.

[0019] The reactor-homogenizer is also connected with the crystal boraxphase separator and liquid radioactive waste containing 20-25 g/l boronacid.

[0020] The liquid radioactive waste runs to a mixer-settling tank thatis connected in its upper part with a reservoir for feeding withalkaline-earth metals salt solutions and in its lower part withseparator for alkaline-earth borates. From the last the crystal phase ofthese alkaline-earth borates are subject to rectification in separatorand after that feeds the packing unit. The liquid radioactive wastecontaining 2-4 g/l boron salts feeds the treated radioactive wastereservoir.

[0021] The other product—crystal phase of borax produced by theseparator connected with the reactor-homogenizer feeds and is dissolvedinto a buffer reservoir for the borax solution, passes through filterand is transported to reactor and separator. After that recrystallizedborax feeds the separator and the mother solution is collected in areservoir and is returned in the interim buffer borax solutionreservoir.

[0022] One part of the recrystallized borax from separator feeds thepacking unit and the other part feeds the reservoir for dissolving onceagain and after that feeds the electrodialysis assembly where solutionsof boron acid and sodium hydroxide are produced.

[0023] The advantages of this method and the installation forradioactive waste treatment according to the present invention consistin the following:

[0024] suitable to use products are being obtained from the radioactivewastes-borax, calcium-magnesium borates, solution of boron acid andsodium hydroxide with environmentally allowed content of radioactiveisotopes;

[0025] the radioactive waste for a long-term storage obtained in resultof this method and installation contains very small quantities of boronacid, e.g. does not take large spaces in the radioactive waste storagearea with non-radioactive products.

[0026] The present invention is explained by the attached technicalscheme of the treatment installation for radioactive waste resulted fromthe operation of nuclear power plants—FIG. 1.

[0027] Marks Explanation Sheet

[0028] The installation consist of the following equipment:

[0029]1—reservoir for radioactive wastes with pH under 5.5

[0030]2—reservoir for radioactive wastes with pH above 5.5

[0031]3—reservoir for correctional additives for pH adjustment

[0032]4—reactor-homogenizer

[0033]5—borax crystal phase and liquid radioactive waste separator

[0034]6—mixer-settling tank

[0035]7—reservoir for solutions of alkaline-earth metals salts

[0036]8—borate (calcium and magnesium) separator

[0037]9—separator

[0038]10—reservoir for treated radioactive wastes with boron saltsconcentration about 2-4 g/l.

[0039]11—buffer reservoir for borax solution

[0040]12—filter

[0041]13—reactor-crystalyzer

[0042]14—recrystallized borax separator

[0043]15—interim reservoir

[0044]16—borax separator

[0045]17—reservoir for dissolving of recrystallized borax

[0046]18—electrodialysis assembly

[0047]19—boron acid solution reservoir

[0048]20—sodium hydroxide solution reservoir

[0049]21—packing unit

[0050] The installation operates as follows:

[0051] Radioactive waste with pH under 5.5 from the reservoir 1 andradioactive waste with pH above 5.5 from the reservoir 2 are mixed inreactor-homogenizer 4 with additives from reservoir 3. In result ofinteraction, the borax is produced in separator 5. The borax crystalphase is transferred and dissolved in buffer reservoir 11, filtered bythe filter 12, transported to the reactor-crystallizer 13 and istransferred in recrystallised borax separator 14. Then it feedsseparator 16 for borax separation and is packed in packing unit 21. Themother solution from separator 14 is collected in the reservoir 15 andfeeds the buffer reservoir 11 one more time.

[0052] The liquid radioactive waste after borax separation fromseparator 5 is transported to mixer-settling tank 6 where it mixes withalkaline-earth salts solutions from reservoir 7. After that the boratecompounds are being separated in separator 8 and refined in theseparator 9 and are packed in packing unit 21. The so obtained insettling tank 6 liquid radioactive waste containing boron salts form 2to 4 g/l through separator 8 feeds the treated radioactive wastesreservoir 10 for storage.

[0053] Recrystallized borax from separator 16 after dissolving inreservoir 17 feeds the electrodialysis assembly 18. After that in thereservoir 19 boron acid solutions with concentration up to 60 g/l and inreservoir 20—solutions of sodium hydroxide with concentrations up to 150g/l are produced.

EXAMPLE FOR IMPLEMENTATION OF THE INVENTION

[0054] The present invention is illustrated by, but is no means limitedto the following examples.

Example 1

[0055] A 1 litter radioactive waste with pH 8.0 containing 35 g/l boronacid (boron salts) is mixed with radioactive waste with pH 10.1 untilthe mixture reaches pH 9.1. Then the borax hard crystal phase separationto the other residual part of liquid radioactive waste 9.0 milliliterssolution of calcium nitrate is added in concentration of 900 g/l. The soobtained non-dissolvable calcium borate is separated from the mixtureand liquid radioactive waste is subjected to concentration until thewaste reaches concentration 2.2 g/l boron acid. Separated calciumborates are subjected several times to washing with water and areseparated as not radioactive product. From the separated andrecrystallized borax a solution is prepared with concentration 20 g/land subjected to electrodialysis. In the electrodialysis assemblyheat-resistant membranes and a current of 0.35 amperes and voltage 5.2Volts are used. The obtained products are: boron acid solution inconcentration of 10 g/l and sodium hydroxide solution in concentrationof 1.5 g/l.

Example 2

[0056] One liter of radioactive waste with pH 10.0 containing 200 g/lboron acid (boron salts) are mixed with radioactive waste with pH 4.0until the mixture reaches pH 8.2. After borax separation as a hardcrystal phase to residual liquid radioactive waste, 9.4 milliliters ofmagnesium-chloride solution is added in concentration of 500 g/l. Thethen obtained dissolvable magnesium borates are separated from themixture and the liquid radioactive waste is concentrated to a 3.4 g/lboron acid.

[0057] Separated borax is set to recrystallisation and from refinedborax a solution is prepared with concentration 25 g/l which is treatedby electrodialysis means of heat-resistant membranes and current of 45Amperes and voltage 55 Volts. The obtained products are: boron acidsolution in concentration 59 g/l and sodium hydrohyde solution inconcentration 150 g/l.

1. A method for treatment of radioactive waste resulting from theoperation of nuclear power plants with pressurized water reactors andboron reactivity regulation including separated collecting of acids thatdo not contain boron salts and alkali that contain boron saltsradioactive spent waters, separate concentration of liquid radioactivewastes with pH under 5.5 and pH above 5.5, that can be characterizedwith the fact that radioactive wastes containing boron acid areconcentrated at pH above 5.5 until reaching of boron acid from 35 to 200g/l and radioactive wastes with pH under 5.5 and do not contain boronacid are concentrated until reaching of total salt content of 400 g/l to500 g/l, after that concentrates are mixed in ratio of recalculatedcontaining in the wastes gram-equivalents sodium hydroxide to boron acidfrom 0.5:1 to 1:2, which ratio leads to producing of mixture with pHfrom 8.0 to 10.1 from which a boron crystallization process isintroduced followed by filtering, washing, recrystallization andliberation of crystal borax radioactively harmless to the environment onthe one hand and on the other hand liberation of filtrate containing20-25 g/l of boron acid, a part of which is mixed with alkaline-earthmetals salts in which process non-solvable, radioactively harmless tothe environment borates of alkaline-earth metals are liberated andliquid radioactive waste containing 2-4 g/l boron acid and the otherpart of the filtrate with content of 20-25 g/l boron acid and boraxsolution with the same content of born acid is treated byelectrodialysis until sodium hydroxide production with concentration upto 150 g/l and solution of boron acid in concentration up to maximum 60g/l.
 2. A method according to claim 1 that can be characterized by thefact that as alkaline salts—earth metals calcium and/or magnesium saltsand/or their mixtures are used.
 3. A method according to claim 1 ischaracterized by the fact, that electrodialysis solution of borax isimplemented by using heat-resistant membranes with a current of 0.2 to45 Amperes and voltage form 5.0 to 55 Volts.
 4. Installation fortreatment of radioactive wastes resulted from the operation of nuclearpower plants that consist of reactor-homogenizer fed from connected tothem reservoirs for radioactive wastes with pH under 5.5, and pH above5.5 as well as a reservoir for pH correcting additives, separator forthe crystal borax phase and liquid radioactive waste, characterized bythe fact that liquid radioactive waste containing 20-25 g/l boron acidseparated by separator (5) feeds the mixer-settling tank (6) to whichupper part is connected reservoir (7) for feeding of alkaline-earthmetal salt solutions and in its lower part the mixer-settling tank (6)is connected to borate separator (8) from which the borate crystal phaseof alkaline-earth borates runs for refining in the separator (9) andafter that feeds the packing unit (21), and liquid radioactive wastecontaining 2-4 g/l of boron salts feeds the reservoir (10) for treatedradioactive wastes, and on the other hand the borax crystal phase thatgoes out form the separator (5) feeds and dissolves in the reservoir(11), is filtered through filter (12), transported to reactor (13) andseparator (14) from which recrystalized borax feeds the separator (16),the mother solution is collected into the interim reservoir (15) and isreturned to the buffer reservoir (11) and one part of recrystalizedborax from separator (10) feeds the packing unit (21) and the anotherpart of recrystalised borax goes for dissolving in the reservoir (17) orhere is submitted solution of borax from the reactor (13) also afterthat the solved borax feeds the electrodialysis assembly (18) from whichafter electrodialysis is submitted boron acid solution in the reservoir(19) and sodium hydroxide solution in the reservoir (20).